You may need to put Gracie on prescription food short term to dissolve the crystals. IMO, Waltham is preferable to Hills.
I would monitor her at home with test strips. You can check for infection and make sure her urine is neutral. Struvite stones form when the urine is too alkaline and infection is present.
Bayer makes the strips, but you can also find generics for about $10 less:
Amazon.com: Multistix Bayer Urine Test Strips 10...
Here's a good article on struvite stones:
Canine Struvite Bladder Stones